Micro-electromechanical systems (MEMS) devices have a wide variety of applications and are prevalent in commercial products. One type of MEMS device is a MEMS varactor (variable capacitor). A MEMS RF varactor may be used for RF filter frequency tuning to enhance the wireless system's capability. A tunable RF filter includes one or more MEMS varactors arranged in the filter circuit. The MEMS varactor is ideal for wireless devices because of their low power characteristics and ability to operate in radio frequency ranges. MEMS RF varactors show their promising applications in cellular telephones, wireless computer networks, communication systems, and radar systems. In wireless devices, MEMS RF varactors may be used for tunable antenna, tunable filter banks, etc.
MEMS varactors may be implemented to provide solutions for achieving capacitance tuning for RF applications, such as tunable filters. Most varactors include a single gap, which limits tuning ratio. Thus, the gap is the same at both capacitor and actuation regions. Such structure has the advantage of simple fabrication. However, the top electrode can only be moved down to approximately one-third of the air gap before the “pull-in” occurs. This causes an abrupt increase of capacitance that cannot be used beyond this point for a continuous tuning application.